1. Field of the Invention
The present invention relates to an impedance matching device used between a high frequency power source and a load. The present invention also relates to a method and a system for analyzing the electric characteristics at an output terminal of an impedance matching device.
2. Description of the Related Art
One of the most important processes performed in producing semiconductor devices or flat panel displays is plasma processing. For generating plasma, a high frequency voltage (RF voltage) of about 100 kHz˜300 MHz is applied to a specially designed plasma chamber.
To maximize the power supply to the plasma chamber, it is necessary to match the impedance of the power source and the impedance of the plasma chamber (“load impedance”), so that the reflection power from the chamber to the power source is minimized. To this end, as shown in FIG. 25, use may be made of an impedance matching device 53 arranged between the power source 51 and the plasma chamber 52. The impedance matching device 53 includes an input detector 61, an output detector 62, a controller 63, an inductor L1 (its inductance is also denoted by L1), and variable capacitors (variable reactance elements, or reactors) VC1, VC2.
Input detector 61 includes an RF voltage detecting unit, an RF current detecting unit and a phase difference detecting unit. The voltage detecting unit and the current detecting unit are provided for detecting RF voltage Vi and RF current Ii both inputted to an input terminal 53a of the matching device 53. The phase difference detecting unit is provided for detecting the phase difference θi between the RF voltage Vi and the RF current Ii. The detection results are sent to the controller 63.
The output detector 62 includes an RF voltage detecting unit, an RF current detecting unit and a phase difference detecting unit. The voltage detecting unit and the current detecting unit are provided for detecting RF voltage Vo and RF current Io both outputted at an output terminal 53b of the matching device 53. The phase difference detecting unit is provided for detecting the phase difference θo between the RF voltage Vo and the RF current Io. The detection results sent to the controller 63.
The controller 63 calculates the input impedance Zi of the matching device 53 by using the following equations (1)˜(3), on the basis of the detected values Vi, Ii and θi. In the equation (1), Ri and Zi denote the resistance component and the reactance component of the input impedance Zi, respectively.Zi=Ri+jXi  (1) Ri=(Vi/Ii)×cos(θi)  (2)Xi=(Vi/Ii)×sin(θi)  (3)
For matching the input impedance Zi and the output impedance Zg of the power source 51, the capacitances of the variable capacitors VC1, VC2 are adjusted (typically the nominal impedance value is 50 Ω). Specifically, the capacitance adjustment can be performed so that the absolute value of Zi (which can be represented as |Zi|) will fall in the prescribed range R. As an alternative, the capacitance adjustment may be performed so that the absolute value of Γi is no greater than a given threshold, where Γi is the reflection coefficient, which is equal to (Zi−Zc)/(Zi+Zc), at the input terminal 53a when the characteristic impedance Zc is 50 Ω.
When the capacitance adjustment is properly performed, the reflection of power at the input terminal 53a is minimized. Accordingly the chamber 52 can receive the maximum power supply.
The impedance of the chamber 52 will be varied by the state of the plasma, for example. Even in this case, the impedance matching can be maintained by the controller 63, which adjusts the capacitances C1, C2 of the capacitors VC1, VC2 on the basis of the detection results supplied from the input detector 61.
The RF voltage Vo, the RF current Io and the phase difference θo are very important numerical factors for knowing the conditions of the plasma treatment. These values are detected by the output detector 62, and then supplied to the controller 63. Based on the supplied data, the controller 63 calculates the output impedance Zo of the impedance matching device 53 by using the following equations (4)˜(6).Zo=Ro+jXo  (4)Ro=(Vo/Io)×cos(θo)  (5)Xo=(Vo/Io)×sin(θo)  (6)where Ro and Xo in the equation (4) are the resistance component and the reactance component of the output impedance Zo, respectively. The calculated Zo, together with the RF voltage Vo, the RF current Io and the phase difference θo detected by the output detector 62, is displayed on the monitor or printed on recoding paper.
In the plasma processing, the power source 51 supplies the plasma chamber 52 with an RF power, which may have a frequency of 13.65 MHz, for example. Thus, unwanted high-frequency components occur in the chamber 52, to be outputted to the impedance matching device 53 together with the wave of the basic frequency (i.e., 13.65 MHz). In other word, some noise is mixed into the basic wave. Under this situation, it is difficult to calculate the precise value of the output impedance Zo (or load impedance Zl) on the basis of the detected RF voltage Vo, RF current Io and the phase difference θo. Furthermore, the level of the RF voltage Vo and the RF current Io at the output terminal 53b of the device 53 can change largely as the impedance Zl of the chamber 52 changes. As a result, the detection accuracy of the output detector 62 falls off, whereby the exact value of the output impedance Zo cannot be obtained.
It is possible to calculate an theoretical output impedance without relying on the actual detecting operation by the output detector 62. In this case, the controller 63 performs calculations on the basis of the following equations (7) and (8), where ω is the angular frequency of the RF power supplied from the power source 51.Ro=Ri×(ωC1)2/B  (7)Xo=[(Ri)2×(ωC1)2+(Xi+ωC1)×ωC1×Xi]/[B+ω(C2−L1)]  (8)whereB=(Ri)2+(Xi+ωC1)2.
However, the obtained theoretical value often fails to reflect the actual value of the output impedance Zo. This is because the above equations do not take into consideration influential factors such as a stray capacitance or inductance component inherent to the physical or electrical structure of the matching device 53.